1. Field of the Invention
This invention relates to the field of communications, and in particular to a switching and bias correction system for direct conversion receivers.
2. Description of Related Art
Direct conversion receivers are commonly used in wireless communication devices, such as cellular telephones. A particular concern in the design of such devices is the interference that each device may cause to each other device in a local environment. For example, in a direct conversion receiver, wherein a local oscillator (LO) signal is mixed directly with the received radio-frequency (RF) signal, leakage from the local oscillator is easily communicated to the RF antenna, and can interfere with other signals in the vicinity of the antenna.
FIG. 1 illustrates an example schematic of a prior art direct conversion receiver system 100. As illustrated, a mixer 150 demodulates an RF signal 110 via a local oscillator 120 to produce a baseband output Vout. Ideally, if the complementary outputs of the local oscillator 120 are true inversions of each other, if the transistors and load resistors are matched, and if the input transconductance is ideally balanced, there will be no leakage of the local oscillator 120 into the RF signal 110. To minimize noise sensitivity and to further minimize the propagation of the local oscillator signal 120 to the RF signal 110, differential circuits are used throughout, thereby cancelling the common-mode leakage or noise.
If the complementary outputs, or phases, of the local oscillator 120 are not truly complementary of each other, the switching points of the transistors in the mixer 150 will differ, and a difference signal at the local oscillator frequency will be created. Although this difference signal is a common-mode signal, an imbalance of the input transconductance at the tail nodes 151, 152 will transform this common-mode signal to a differential-mode signal, which will be propagated back to the RF signal 110, and possibly emanated from the RF antenna at the local oscillator frequency.
Even with true complementary outputs of the local oscillator 120, random mismatches of component values within the mixer 150 may affect the DC offset of one or more of the transistors, such that the crossover points in the individual transistor pairs is offset from the instant where the complementary outputs of the local oscillator 120 cross. Such an offset will produce a mark:space ratio that is no longer 1:1, and the signals at the tail nodes 151, 152 will not be matched. This mismatch will produce a differential-mode signal at the local oscillator frequency that is propagated to the RF signal 110, and possibly emanated from the RF antenna, regardless of the balance of the input transconductance at the tail nodes 151, 152.